POST between c# and vue

---EDITED QUESTION:---

I am building my very first website with c# web application as backend and Vue as frontend. My GET and DELETE works fine, but when I want to run POST I get problems I cant fully understand how to fix..

According to my teacher my Axios connection looks fine and the problem is only in my backend..

What I think I am trying to do is to somehow bind the models so they speak to eachother?

My datamodel is Sale.cs and my viewmodel (from Vue) is called SaleVM.cs

Sale.cs:

using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;

namespace Models
{
    public class Sale
    {
        public enum Status
        {
            Started,
            Ongoing,
            Done,
            Removed
        }

        [Key]
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public int Id { get; set; } //++
        [Required]
        public string CustomerNumber { get; set; }
        public int UserId { get; set; } //fk
        [Required]
        public string YourReference { get; set; }
        public DateTime DateSold { get; set; }
        public DateTime DateCreated { get; set; }
        public DateTime? DateDone { get; set; }
        public DateTime? DateEdited { get; set; }
        public Status StatusId { get; set; }
        public DateTime? DateDelivered { get; set; }

        public virtual ICollection<SaleArticle> SaleArticles { get; set; }
        public virtual User User { get; set; }

    }
}

SaleVM:

using System.Collections.Generic;

namespace Models
{
    public class SaleVM
    {
        public enum Status
        {
            Started,
            Ongoing,
            Done,
            Removed
        }

        public int Id { get; set; }
        public string Reference { get; set; }
        public DateTime DateSold { get; set; }
        public DateTime DateCreated { get; set; }
        public Status StatusId { get; set; }
        public int UserId { get; set; }

        public virtual ICollection<ArticleRow> ArticleRows { get; set; }
        public virtual SelectedCustomer SelectedCustomer { get; set; }

    }
}

I think i have made them incorrectly because I have only guessed how to write them.

and lastly my SaleController:

{
    [Route("api/[controller]")]
    [ApiController]
    public class SaleController : ControllerBase
    {
        private readonly DataContext _context;

        public SaleController(DataContext context)
        {
            _context = context;
        }

        // POST: api/Sales/5
        [HttpPost]
        public async Task<ActionResult<Sale>> PostSale(SaleVM saleVM)
        {
          string customerNumber = (saleVM != null && saleVM.SelectedCustomer != null && Convert.ToInt32(saleVM.SelectedCustomer.CustomerNumber) > 0) ? saleVM.SelectedCustomer.CustomerNumber : "";

          List<SaleArticle> saleArticles = new List<SaleArticle>();
            if (saleVM != null && saleVM.ArticleRows != null && Convert.ToInt32(saleVM.ArticleRows) > 0)
            {
                foreach (var item in saleVM.ArticleRows)
                {
                    saleArticles.Add(new SaleArticle()
                    {
                        ArticleNumber = item.ArticleNumber,
                        Price = item.SalesPrice,
                        Quantity = item.Quantity,
                        Description = item.Description,
                        Id = item.Name
                    });
                }
            }
            //foreach (var saleVM in SaleVM)
            //{
            Sale sale = new Sale();
            {
                sale.CustomerNumber = saleVM.SelectedCustomer.CustomerNumber;
                sale.YourReference = saleVM.Reference;
                sale.SaleArticles = saleArticles;
                sale.DateCreated = saleVM.DateCreated;
                sale.DateSold = saleVM.DateSold;
                sale.StatusId = (Sale.Status)saleVM.StatusId;
                sale.UserId = saleVM.UserId;
            };

            _context.Sales.Add(sale);
            //}

            await _context.SaveChangesAsync();

            return CreatedAtAction(nameof(GetSale), new { id = saleVM.Id }, saleVM);
        }
    }
 }

I dont get any errormessages in visual studio, but when I try the code out in postman i get statuscode 500 Internal Server Error:

System.NullReferenceException: Object reference not set to an instance of an object. at E37SalesApi.Controllers.SaleController.PostSale(SaleVM saleVM) in C:\Users\fadaka\source\repos\E37SalesApi\Controllers\SaleController.cs:line 47 at lambda_method(Closure , Object ) at Microsoft.Extensions.Internal.ObjectMethodExecutorAwaitable.Awaiter.GetResult() at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.AwaitableObjectResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments) at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.g__Awaited|12_0(ControllerActionInvoker invoker, ValueTask`1 actionResultValueTask) at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.g__Awaited|10_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted) at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Rethrow(ActionExecutedContextSealed context) at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted) at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeInnerFilterAsync() --- End of stack trace from previous location where exception was thrown --- at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.g__Awaited|19_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted) at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope) at Microsoft.AspNetCore.Routing.EndpointMiddleware.g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger) at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context) at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

I do understand what it says, its just I cant really understand how to fix it and get my POST method to fully work.

I hope I am clear with my problem

I am very new to programming and find it very hard to find information online that is on a level that a noob like me can understand, especially when it comes to c# and vue working together.